I know the difference and in Florida the plastic case is the one I would recommend because, even the 3R case will be rusted out pretty fast.
I was just disappointed that there wasn't more guidance on the box or the installation label. Of course Home Depot, where the installer ("maintenance" sort of guy) bought this, sold it is "outside" based on the box.
I will take this to Intermatic.com and see what they say.
